1. Field of the Invention
The present invention relates generally to the field of optical systems, and specifically, to an anti-aliasing diffractive aperture for reducing aliasing effects.
2. Background Information
Optical systems that discretely sample images on detector arrays produce aliasing effects in the displayed image. Aliasing occurs in detector arrays when the spatial frequencies in the image are higher than the detector array's Nyquist frequency (i.e., half the sensor sampling frequency). Color Filter Arrays ("CFAs") integrated into discretely sampled detector arrays introduce color-aliasing effects. An example of aliasing effects in a monochrome (black and white) TV image are Moire effects that cause an image having a periodic structure to move and change patterns when the image moves relative to a charge coupled device ("CCD") detector array and has dimensions commensurate with the pixel dimensions of the detector array. An example of color aliasing effects in a CFA CCD detector system is the similar Moire effects that cause an image having a periodic structure to move, change patterns, and change color when the image moves relative to the detector array and has dimensions commensurate with the pixel dimensions of the detector. For example, when a building, having very narrow columns, is carefully observed on a TV using a CFA detector system in a camera, some columns will have a red tint, some a green tint, and some a blue tint. As the camera moves, the tinted pattern shifts from one column to another.
One solution for eliminating or minimizing aliasing effects involves the addition of a quartz plate in front of the detector array. The quartz plate blurs and reduces the sharpness of the image just the right amount to minimize such aliasing effects. However, quartz plates are very expensive, virtually costing as much as a lens system. Another solution involves the use of molded structures near the aperture stop of a lens system. These structures typically use a single, very weak (i.e., having a height of a few wavelengths of light), four-sided pyramid to form four displaced images at the detector array surface. Because each of these four images are formed by a section of the aperture that looks like one-quarter of a pie, the image structure is a function of the size of the aperture stop, the object distance, and the zoom position. In essence, this pyramidal feature divides the incident wavefront from a point object into four pie-shaped wavefronts that converge to four blurred images at the detector. However, this type of anti-aliasing feature is undesirable because the amount of anti-aliasing required changes as the size of the aperture stop, the object distance, and the zoom position changes.
Accordingly, there is a need in the art for a method and apparatus for minimizing aliasing and color aliasing commonly associated with digital cameras that discretely sample an image produced by an optical system.